A clathrate hydrate refers to a crystalline compound wherein guest molecules are physically trapped inside a three-dimensional lattice structure formed by hydrogen-bonded host molecules without a chemical bonding. When the host molecule is a water molecule and the guest molecule is a small-molecular-weight gas molecule such as methane, ethane, propane and carbon dioxide, it is called a gas hydrate.
The gas hydrate was first discovered in 1810 by Sir Humphry Davy of England. He reported during his Bakerian Lecture to the Royal Society of London that, when chlorine reacts with water, a compound resembling ice is formed, but the temperature thereof is higher than 0° C. Michael Faraday first discovered in 1823 that a gas hydrate is formed by a reaction of 10 water molecules with one chlorine molecule. Until now since then, the gas hydrate has been continuously studied as one of phase-change materials (PCMs). The main subjects of the study include phase equilibrium and formation/dissociation conditions, crystal structure, coexistence of different crystals, competitive compositional change in the cavity, etc. Besides, various detailed researches are being conducted in microscopic and macroscopic aspects.
At present, it is known that about 130 kinds of guest molecules can be trapped in the gas hydrate. Examples include CH4, C2H6, C3H8, CO2, H2, SF6, etc. The crystal structure of the gas hydrate has a polyhedral cavity which is formed by hydrogen bonded water molecules. Depending on the kind of the gas molecule and the condition of its formation, the crystal structure may vary to have a body-centered cubic structure I (sI), a diamond cubic structure II (sII) or a hexagonal structure H (sH). The sI and sII structures are determined by the size of the guest molecule and, in the sH structure, the size and the shape of the guest molecule are important factors.
The guest molecule of the gas hydrate naturally occurring in the deep sea and permafrost areas is mainly methane, and it has received attention as an environment-friendly clean energy source due to a small amount of carbon dioxide (CO2) emissions during combustion. Specifically, the gas hydrate may be used as an energy source to replace traditional fossil fuels and may also be used for storage and transportation of solidified natural gas using the hydrate structure. Further, it may be used for separation and storage of CO2 to prevent global warming and may also be beneficially used in seawater desalination apparatuses to dissociate gases or aqueous solutions.
In technologies utilizing the gas hydrate such as storage and transportation of solidified natural gas, seawater desalination, etc., the method of preparing the gas hydrates at relatively low pressure with high speed is an important factor in commercialization.
In the conventional methods, the reaction between the materials introduced into the reactor is facilitated by further adding a reaction promoter or by increasing the efficiency of heat exchange using a stirrer, a cooling jacket, etc., equipped inside or outside the reactor in order to promote the formation of the gas hydrates. However, the additional use of the promoter or the devices leads to increased cost and it is still practically difficult to maintain the gas hydrate formation rate enough to provide satisfactory cost-effectiveness and productivity at low temperature.